1. Field of the Invention
The present invention relates to a high strength ceramic composite composed mainly of Si.sub.3 N.sub.4 and containing SiC whiskers and a method of producing the same.
2. Description of the Prior Art
There is a great difficulty to obtain sintered bodies of Si.sub.3 N.sub.4 having a high density when Si.sub.3 N.sub.4 is used alone. This is due to the fact that constitute atoms of Si.sub.3 N.sub.4 have a small self-diffusion coefficient and Si.sub.3 N.sub.4 has strong covalent properties. Usually, a sintering aid such as MgO, Y.sub.2 O.sub.3 or Al.sub.2 O.sub.3 has been added to obtain high density sintered bodies. However, in such a case, the sintering aid precipitates as a glass phase along grain boundaries and many problems, such as deterioration of hardness and strength, has arisen in a high temperature range of 1000.degree. C. or higher due to the softening of the glass phase.
In order to improve high temperature properties, various ways of achieving sintered bodies of high density have been studied without adding a sintering aid.
For example, Shimada, et al. reported in Yogyo Kyokai-shi, Vol. 89, No. 4, pp. 197 to 203 (1981) that sintered bodies having a relative density of 99% or higher can be obtained by a superhigh pressure sintering process. Also, such high density sintered bodies have been reported by Honma, et al., in Yogyo Kyokai-shi, Vol. 95, No.2, pp. 229 to 234 (1987) and Nezuka, et al., in Digest of 25th Basic Ceramic Symposium, pp 29 (1987) in which a hot isostatic pressing (HIP) is employed. According to these reports, it has been found that the sintering aid-free Si.sub.3 N.sub.4 sintered bodies are not subject to deterioration of their properties at high temperatures. However, the Si.sub.3 N.sub.4 sintered bodies without the addition of sintering aids are disadvantageous in that their strength and toughness are both low. In Si.sub.3 N.sub.4 sintered bodies containing a sintering aid, .beta.-Si.sub.3 N.sub.4 grows as needle-like grains and the grains thus grown tangled together, thereby resulting in sintered bodies having a high strength and a high toughness. In contrast to this, in the case of sintered bodies without addition of sintering aids, .beta.-Si.sub.3 N.sub.4 grows as a granular form and needle-like grains are not formed. Consequently, strength and toughness are reduced.